Due to the increasing strictness of current or future standards, pollutant emissions must be reduced, particularly during cold starting, in the first minutes in which the engine is running. It is known that most pollutants (80%) are released during this time, when the conversion catalyst located in the exhaust line has not yet been sufficiently heated by the exhaust gases to be effective.
It is now only too well known that one of the major obstacles to cleanup of automobiles occurs during the temperature rise phase following a cold start.
There are several reasons for this, of which the most important is the enormous increase in the viscosity of lubricants when cold, which may exceed 1000 cSt below 0.degree. C. resulting in very high friction levels in the engine, bringing about equally high consumption levels. This is aggravated by the difficulties with supply at low temperatures, and the poor quality of the carbon-containing mixtures arising from inadequate atomization and evaporation of the fuel in the intake air, which is compensated, albeit inadequately, by enriching the air/fuel mix. Overconsumption is of course accompanied by high emissions of basic pollutants, carbon monoxide, and unburnt compounds, resulting from overly-rich mixtures and incomplete combustion, which the exhaust catalyst cannot eliminate until its activation temperature is reached, which takes several minutes.
Moreover, improvements in the design of combustion chambers have brought about a real increase in thermodynamic efficiency in recent engines, so that less heat is transferred through the cylinder walls and hence a further difficulty in heating the engine, and thus the passenger compartment of the vehicle, by exploiting the heat contained in the coolant in a natural manner.
Numerous studies have been directed at the problem of cold starts in automobiles. The solutions are essentially of two types: those outside the engine and those directly involving the engine.
Solutions involving the engine include for example pre-evaporation of fuel, improving atomization of the fuel, or homogenizing the incoming fuel. As an example, patent FR 2,706,184 describes such a fuel vaporization device.
Solutions "outside" the engine relate in particular to the exhaust line, specifically reducing the time necessary to activate the catalyst or catalysts located in the exhaust.
A known method is to recover the catalyst from a layer designed to reduce the temperature at which the catalyst begins to become active.
Another solution may be to inject additional air upstream of the catalyst, which thus receives additional energy and heats more rapidly.
Electrical heating of the catalyst may also be considered as for example in patent EP-B1-605479. This solution is indeed effective because it is totally focused on the catalytic muffler. However, this system is an energy consumer because it consumes a minimum of three or four kilowatts from the battery. Thus an additional battery must be provided. The energy balance is incompletely satisfactory and the overall efficiency is very low.
Thus, recent developments have endeavored to reduce the electrical capacity required for heating the catalyst.
The solutions in this direction consist of choosing an appropriate catalytic element and placing it in the right spot.
Other technological solutions have been considered, such as preheating the catalytic muffler with hot gases generated by a burner located upstream of said muffler. Patent WO 95/14852 describes an arrangement of this type. The heating power of such a unit is high: approximately 15 kW, so that the catalyst can be activated in approximately twenty seconds. The advantage is the efficiency of this system: approximately 100%.
However, one of the problems of this type of solution is that it requires a burner whose sole function is to heat the catalyst or catalysts.
In addition, both for electrical heating and burner heating, a problem of premature and/or unpredictable catalyst aging may arise due to the sharp heat stresses placed on the catalyst.
The present invention remedies in particular the problems of the prior art listed above.